SOLVENT EFFECTS ON THE ABSORPTION MAXIMA OF FULLERENES C-60 AND C-70

Absorption spectra of fullerenes C-60 and C-70 have been recorded betw een 200 and 700 nm in liquid n-alkanes at room temperature. Solvent sh ifts (-p) of band maxima vary from 900 cm(-1) (the 636 nn band in C-70 ) to 18 000-19 000 cm(-1) (the 209 nm band in C-60 and the 210 nm band in C-70) per unit Lorentz-Lorenz function phi(n(2)) = (n(2) - 1)/(n(2 ) + 2), where n is the refractive index at the Na D line. Changes of s tatic polarizabilities (Delta alpha) have been calculated from the Bak hshiev formula by equating the Onsager cavity radii to 0.75 and 0.8 nn for C-60 and C-70, respectively. The estimated ba values lie in the i nterval between 10 and 15 Angstrom(3) for weak transitions (400-650 nm ) and reach magnitudes as high as 150-200 Angstrom(3) for the stronges t transitions at 210 nm. The dispersive shift of the equally intense 2 56 nm band in C-60 and 235 nm band in C-70 yields Delta alpha values o f 41 +/- 2 and 79 +/- 10 Angstrom(3), respectively. Allowed transition s between 300 and 400 nn possess a relatively small solvent shift and Delta alpha: -p = 2000-2500 cm(-1) and Delta alpha = 20-25 Angstrom(3) . Published values of absorption and fluorescence maxima in cold jets, vapors, solid inert gases, solvent and polymer glasses, and doped and neat crystals have been plotted as a function of phi(n(2)). A purely thermal bathochromic shift of 50-100 cm(-1) between low (4-77K) and am bient temperatures could be established for 620, 404 (C-60), and 636 n m (C-70) bands. The large bandwidth of allowed transitions(<400 nm) ha s been accounted for in terms of a much stronger Franck-Condon couplin g to higher harmonics of intramolecular vibrations than that in planar aromatic hydrocarbons.